1. Field of the Invention
The present invention relates to self-aligning fastening devices and means for installing same.
2. Description of the Prior Art
Since the advent of high performance commercial and military aircraft, the industry has faced the problem of providing rapid access to internally installed equipment for the purpose of inspection, service, maintenance and/or replacement. In doing this the nature of aircraft loads must be considered. The major loads imposed on the structure are those due to flight, landing and handling. In modern aircraft these are carried through the aircraft skin and, using the wing as an example, vary from low near the tip to high near the fuselage.
The constraints imposed on the designer include weight, cost, time and reliability. Historically, he has had to make a choice between two basic methods of handling the loads in the area of the cutout in the skin created by the need for internal access. He may elect to use a thin gauge non-stressed panel in conjunction with added internal structure designed and located to carry the primary bending loads around the cutout. This usually takes the form of a machined forging. The removable access panel is light since it carries only local air loads. Oversize holes may then be used in both the panel and the internal structure, together with a floating receptacle mounted on the internal structure. This design generally accepts misaligned holes and is usually used with a quick release type fastener. Tooling and labor costs for the panel installation are lower since only non-precision drill fixtures are required. A disadvantage to this method is the added cost and weight of the machined forging and the added weight it presents.
The alternate method is to use a load bearing access panel, usually of the same thickness as the surrounding skin, with close fitting fastener holes in both the panel and the adjacent structure which are connected with close tolerance fasteners. This technique provides the necessary bearing area and the intimate contact between all the load carrying members necessary to transfer the high shear and bending loads involved. This latter approach, on the other hand, requires master tooling, precision drill fixtures and a high degree of labor skill, while providing weight saving.
The costs of both types of panel installation are initially about the same. However, when considered in terms of the "Life Cycle Costing" system as used by the military in arriving at the total cost of the original acquisition and subsequent support and maintenance of an aircraft fleet over its expected life, the costs of the stressed access panel become exorbitant. This can easily be explained.
Experience has shown that minute shifting or working of the aircraft structure, when subjected to flight/landing/handling loads, very easily uses up the slight clearance between close fitting fasteners and holes. As an example, using a 0.25 inch diameter screw, the total diametral clearance is ##EQU1## With fits like this, all too frequently when a panel is removed, the load is relieved and one or more of the fasteners cannot be replaced. The standard repair technique is then to drill the holes out to accept the next larger size fastener. This can usually be done only once for a given hole before running out of acceptable edge distance for the fastener. When this occurs, the usual remedy is to requisition a replacement panel, preferably a blank, and completing it on the spot to fit the structure. It is this condition that necessitates the procurement, storage and disbursement of a high number of spare access panels for the full life span of the aircraft fleet. It is readily apparent that this is not only expensive in terms of parts, storage facilities, labor and time, but also results in grounded aircraft, useless until they are repaired.
The present invention eliminates this basic problem by permitting the original design to accept mating holes in both the access panel and the internal structure that are grossly misaligned, either initially or through subsequent deformation, while retaining intimate contact between all load carrying members. This is rendered possible through the use of a powered tool which, in one sequence, drives the nested bushings to virtually all points of eccentric alignment.